Pyrrole carbonitrile and nitropyrrole compounds useful as acaricides, insecticides and mollusoicide s are described in copending patent application Ser. No. 430,601, filed on Nov. 6, 1989 which is a continuation-in-part of patent application Ser. No. 279,909, filed on Dec. 5, 1988. Molluscicidal 2,4,5-tribromopyrrole-3-carbonitrile and its preparation via bromination of pyrrole-3-carbonitrile are described in the above-said patent applications. However, synthesis of pyrrole-3-carbonitrile is difficult to achieve. Literature methods such as that reported by A. M. van Leusen, et al, Tetrahedran Letters 5337(1972) report yields of 10% or less.
An alternative route to obtain pyrrole-3-carbonitrile is described by H. J. Anderson et al, Canadian Journal of Chemistry, 59, 2673(1981) and Synthetic Communications, 17, 401 (1987) and involves the separate steps of hydrolysis of 2-trichloroacetylpyrrole-4-carbonitrile give 4-cyanopyrrole-2-carboxylic acid, and conversion of the carboxylic acid to pyrrole-3-carbonitrile via a decarboxylation procedure involving a copper chromite catalyst in hot quinoline. However, this procedure may be difficult to control and complex in work-up on a commercial scale.
It is an object of the present invention to provide an efficient and easily controlled process for the preparation of molluscicidal 2,4,5-tribromopyrrole-3-carbonitrile compounds via a single step deacylative bromination of the 2-trihaloacetylpyrrole-4-carbonitrile precursors.